SteveA wrote:
does anyone have a link to any of the studies that supposedly showed the results that led to the "125kVA rule"?
I recently ran a study in SKM with the option set to calculate the values and noticed that several buses below a 112.5KVA 480-208 transformer resulted in 18cal after the protective device 2 second cutoff.
I know the IEEE calcs at those levels are supposedly very conservative, but from "<1.2cal" to 18cal is a much bigger jump than I was expecting.
From IEEE 1584: “It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box. In all other tests with that box and the 305 mm × 368 mm × 191 mm box, the arc blew itself out as soon as the fuse wire vaporized”.
Furthermore, it provides the following guidance: “While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it will work and will yield conservative results. The arc-flash hazard need only be considered for large 208 V systems: systems fed by transformers smaller than 125 kVA should not be a concern.”
So the conclusion here is that it is barely sustainable at 208 V although the results are most likely less in the real world and certainly if there's not much of a transformer or lower voltages, the conclusion at that time is that the arc is certainly not sustainable.
IEEE 1584 empirical equations ASSUME that an arc is sustainable and that the arc is stable. At somewhere below 250 VAC, sustainability is seriously suspect. Even if it will arc, the arc is often weak and does not even get close to the value determined through IEEE 1584 empirical equation. EPRI and others have also done extensive testing on real world equipment and has never achieved anything about 3.2 cal/cm2 so in the IEEE C2 (NESC) table based approach they give a maximum value of 4 cal/cm2.
The inherent problem though is that so far developing any kind of trend or formula for "sustainability" of an arc has been elusive. There are cases where even at 480 V arcs are not sustainable but sustainability in general is hard to achieve below around 250 VAC. By way of example a 130 VDC arc has been tested up to 20 kA. The standard arc gap had to be reduced to 1/4". Under those extreme conditions the arc lasted for 80 milliseconds maximum, not unlike the original 1584 test bank with a single case at 208 VAC. An AC arc is not a continuous arc so we should expect sustainability to be much worse.
There have been tests though that have exceeded 1.2 cal/cm2 at 208 VAC using a box-barrier model and different size boxes and smaller gauge "fuse" wires using short circuit current similar to that achievable with 125 kVA. So there is most certainly a danger at 208 VAC. That much should be evident with the EPRI real world testing at 3.2 cal/cm2. But none of these tests are anywhere near a calculated number of 18 cal/cm2 and I would state pretty strongly that absent any evidence of incident energy that high from any actual laboratory testing at that voltage that it is either highly unlikely or impossible to get that high. I'll certainly concede that up to 4 cal/cm2 is possible but there's no way that 18 cal/cm2 is even possible no matter what some empirical formula estimates.
There's a paper on low voltage arcs that ETAP put out and in the articles section on this forum I also put one there that summarizes a lot of what's available on the subject. Suffice to say that despite what it claims, IEEE 1584 empirical calculation probably isn't really valid below 300 VAC or in the best case it might upper bound things but the upper bound is pretty loose.
